Powder thermosetting compositions for preparing coatings with low gloss finish

ABSTRACT

The invention concerns powder thermosetting coating compositions comprising (a) an amorphous polyester containing carboxyl groups, rich in isophthalic acid and neopentylglycol, optionally branched with a polycarboxylic acid or a polyol containing at least three functional groups; (b) a semicrystalline polyester containing carboxyl groups prepared from one or several saturated aliphatic dicarboxylic acids with linear chain, and from a saturated aliphatic diol with linear or cyclic chain optionally branched with a polycarboxylic acid or with a polyol containing at least three functional groups, and having a melting point (Tm) of at least 40° C. and an acid value of 5 to 50 mg of KOH/g, and (c) a cross-linking agent. Said compositions are useful for preparing powder paints and varnishes providing excellent quality coatings whereof the brilliance, measured at an angle of 60°, according to the ASTM D 523 standard is always less than 50%, that is half-gloss or matt coatings.

This application is a national stage filing of International ApplicationNo. PCT/BE98/00200, filed Dec. 16, 1998, the content of which isincorporated herein by reference. This application claims benefit ofpriority under 35 U.S.C. §119 to Belgian patent application no. 9701039,filed on Dec. 18, 1997, the content of which is also incorporated byreference herein.

DESCRIPTION

The present invention relates to thermosetting powder compositionscomprising, as binder, a blend of an amorphous polyester, of asemicrystalline polyester and of a crosslinking agent, whichcompositions provide, by curing, low-gloss coatings, particularly mattcoatings.

The invention also relates to the use of these compositions for thepreparation of powder paints and varnishes which provide low-glosscoatings, as well as to the low-gloss coatings obtained from thesecompositions.

At the present time, thermosetting powder coating compositions arewidely employed as paints and varnishes in order to form durableprotective coatings on the most varied of objects. Powder coatingcompositions have many advantages over coating compositions that are inthe form of solutions in an organic solvent; on the one hand, the safetyand environmental problems caused by solvents are completely eliminatedand, on the other hand, whereas solvent-based coating compositions havethe drawback of only being able to be used partially—in certain types ofapplication, only 60% or less of the coating composition applied comesinto contact with the substrate and that part which does not come intocontact with the substrate cannot be recovered—power coatingcompositions are used 100%, given that only the powder in direct contactwith the substrate is retained by the latter, the excess powder being,in principle, entirely recoverable and reusable. This is why thesepowder compositions are preferred over coating compositions that are inthe form of solutions in an organic solvent.

Powder coating compositions generally contain a thermosetting organicbinder and, optionally, fillers, pigments, catalysts and variousadditives for tailoring their behaviour to their use.

These thermosetting powders are prepared in the following manner. Thepolyester or polyesters, the crosslinking agent, the optional catalyst,the pigments, the fillers and other additives are dry blended at ambienttemperature in the required proportions in order to obtain a powderpaint or varnish. The blend thus obtained is put into an extruder inorder to produce melt homogenization therein at a temperature generallybetween 80 and 150° C. The blend leaving the extruder is left to cooland then ground and screened in order to obtain a powder having thedesired particle size of between 10 and 150 micrometres.

The powder paints and varnishes thus obtained are applied in a mannerknown per se by means of an electrostatic or triboelectric spray gun orusing the technique of fluidized-bed deposition on the object to becoated. The object thus coated is then heated in an oven where themelting and crosslinking of the binder is carried out at hightemperature.

The cured coatings obtained from thermosetting powder coatingcompositions must have a smooth, uniform and defect-free appearance andmust especially be free of any “orange peel”; they must have goodmechanical and chemical properties and good weatherability.

Furthermore, it is essential that the powder coating compositions remainin the form of freely-flowing powders for a long enough period aftertheir manufacture and their packaging, without reagglomerating duringtransportation and storage.

The thermosetting powder coating compositions commonly used andcommercially available contain, as binder, a blend of an amorphouspolyester containing carboxyl or hydroxyl groups having a glasstransition temperature (T_(g)) of between 45 and 80° C. with across-linking agent having functional groups capable of reacting withthe carboxyl or hydroxyl groups of the polyester.

The amorphous polyesters used in these compositions are those obtainedfrom aromatic dicarboxylic acids, such as terephthalic acid orisophthalic acid, and optionally from aliphatic dicarboxylic acids, suchas adipic acid, and from various polyols, such as neopentylglycol,ethylene glycol, trimethylolpropane, etc.

These amorphous polyester-based compositions are storage-stable andproduce coatings of excellent quality, inter alia a surface without anyvisible defects, and good mechanical properties. Among thesecompositions, amorphous polyester-based compositions rich in isophthalicacid provide coatings which are highly valued for their excellentperformance in outdoor exposure.

However, after curing, these amorphous polyester-based compositionsprovide coatings with a very high gloss. The gloss, measured at an angleof 60°, according to the ASTM D 523 standard, is very often greater than90%.

Powder coating compositions whose binder contains a semicrystallinepolyester have also already been proposed.

Thus, in European Patent 521,992, the binder proposed consists of ablend of at least one semi-crystalline polyester having an acid numberof 10 to 70 mg of KOH per gramme and of at least one amorphous polyesterhaving a glass transition temperature (T_(g)) of at least 30° C. and anacid number of 15 to 90 mg of KOH per gramme, with a crosslinking agentwhich may be an epoxidized compound, a compound containing activatedthiol or hydroxyl groups or an oxazoline.

Semicrystalline polyesters are characterized by one or more glasstransition temperatures (T_(g)) not exceeding 55° C. and a sharp meltingpoint of 50 to 200° C.

According to that patent, semicrystalline polyesters differ fromamorphous polyesters by the fact that semicrystalline polyesters have aheterogeneous morphology (they contain a mixture of phases), are opaqueand white in colour at ambient temperature, have a low melt viscosity,are more insoluble in organic solvents and have a very high structuralregularity. The semicrystalline polyesters described and used in theillustrative embodiments of that patent are those obtained bypolycondensation from dicarboxylic acids containing an aromatic oraliphatic ring, such as terephthalic acid and1,4-cyclohexanedicarboxylic acid, and from saturated aliphatic diolswith a linear chain, such as 1,6-hexanediol and 1,10-decanediol;further-more, aliphatic dicarboxylic acids with a linear chain, such asadipic acid, succinic acid or 1,12-dodecanedioic acid, are addedthereto. The amorphous polyesters used are polyesters containingcarboxyl groups usually employed in powder paints and varnishes; thesepolyesters may be rich in isophthalic acid for the purpose of obtainingoutdoor coatings having good weatherability.

According to that patent, the presence of the semicrystalline polyesterin the binder gives a coating having an excellent overall appearance,free of “orange peel”, and improved mechanical properties.

However, as shown in Table I at the end of the description of thatpatent, the coatings obtained from these compositions all have a veryhigh gloss; the gloss, measured at an angle of 60°, according to theASTM D 523 standard, may vary between 82 and 87%.

U.S. Pat. No. 5,373,084 proposes thermo-setting powder coatingcompositions which comprise, as binder, a blend of a particularsemicrystalline polyester, of an amorphous polyester and of across-linking agent. The particular semicrystalline polyesters proposedin that patent are those obtained by the esterification of a saturatedaliphatic dicarboxylic acid having a linear chain, mainly1,12-dodecanedioic acid, with a saturated aliphatic diol having a linearchain, mainly 1,6-hexanediol, and by optionally incorporating atrifunctional polyol, such as trimethylolpropane or glycerol, or atrifunctional polycarboxylic acid, such as trimellitic acid, in order toobtain branched polyesters. These semicrystalline polyesters have anacid number or hydroxyl number of approximately 20 to 120, preferablyapproximately 30 to 80 mg of KOH per gramme and a melting point of 40 to200° C., preferably 60 to 150° C. The amorphous polyesters used areamorphous polyesters containing conventional hydroxyl or carboxylgroups; these polyesters preferably have a glass transition temperature(T_(g)) of at least 50° C. and an acid number or hydroxyl number ofapproximately 25 to 80 mg of KOH per gramme. However, it will be notedthat the only amorphous polyesters described and used in that patent areamorphous polyesters rich in terephthalic acid, the acid constituent ofwhich contains at least 50 mol % of terephthalic acid and the alcoholconstituent of which contains at least 50 mol % of neopentylglycol andup to 10 mol % of trimethylolpropane; on the other hand, amorphouspolyesters rich in isophthalic acid are not mentioned in that patent.

The particular semicrystalline polyesters proposed in that patent actabove all as reactive plasticizers. In the illustrative embodiments, itis shown that when the powder composition contains a small amount of theplasticizing semicrystalline polyester (at most 10% by weight calculatedwith respect to the total weight of the amorphous and semicrystallinepolyesters), the composition provides coatings having improvedproperties, in this case a smooth appearance with little “orange peel”,a gloss measured at an angle of 60°, according to the ASTM D 523standard, of 96 to 97%, a good pencil hardness and excellent mechanicalproperties. However, just like European Patent 521,992 mentioned above,U.S. Pat. No. 5,373,084 does not disclose the possibility of obtainingcoatings having a low gloss, for example coatings whose gloss measuredat an angle of 60°, according to the ASTM D 523 standard, would be lessthan 50%.

However, there is an increasing need to be able to have availablethermosetting powder paints and varnishes which provide coatings havinga low gloss, such as, for example, satin or semi-gloss coatings or mattcoatings, which can be used inter alia for the coating of certainaccessories in the motor-vehicle industry, such as wheel rims, bumpers,etc., or else for the coating of metal beams and panels used in theconstruction industry.

Various methods have already been proposed for obtaining powder paintsand varnishes which provide coatings having a low gloss. However,experience has shown that it is difficult to produce a paint or varnishcapable of providing, under the usual extrusion and curing conditions, amatt or satin finish completely reliably and reproducibly. According toone of these methods, one or more special flatting agents, such assilica, talc, chalk and metal salts, are introduced into the powdercomposition in addition to the binder and the conventional pigments.However, the reduction in gloss is often insufficient and a pronounceddeterioration of the properties of the paint coatings is observed, suchas, for example a lack of adhesion to metal substrates. In order toremedy these drawbacks, European Patent 165,207 proposes to incorporatewaxes, for example a polyolefin wax, and metal salts (for example, zinc2-benzothiazolethiolate) into thermosetting powder compositions based onpolyesters terminated by carboxyl groups and on epoxidized compounds,such as triglycidyl isocyanurate. Likewise, U.S. Pat. No. 4,242,253proposes, as additives, calcium carbonate and finely dividedpolypropylene particles in order to produce low-gloss coatings. Thedrawback of this system is that the inorganic fillers, oftenincorporated in large amounts, may damage the extruders used forpreparing the powders and impair the desired surface appearance of thecoating obtained, which often has a rough and irregular appearance.Moreover, the waxes that are added easily migrate to the surface,thereby causing unacceptable variations in the degree of mattness of thecoating as it undergoes natural ageing. Furthermore, the fact of havingto add additional fillers in large amounts incurs an extra expense and,in itself, constitutes a drawback.

According to U.S. Pat. No. 3,842,035, it is known to produce a mattfinish by using a coating composition obtained by dry blending twothermosetting powder compositions which were extruded separately. One ofthem is a slowly-curing composition (a long gel time) and the other arapidly-curing composition (a short gel time). Using this system, it ispossible, after curing, to obtain a matt coating without it beingnecessary to add a special matting agent to the powder composition. Themajor drawback of this system is that it requires dry blending largeamounts of already-formulated powders, something which is not easy todo, especially on an industrial scale. In addition, as this blendingcannot be carried out continuously, but only in batches of powder, it isnot easy to obtain the same degree of mattness in the coatings whengoing from one batch of powder to another. Finally, the powder which isrecovered after a first spraying operation and reused as paint cannothave the same composition as the powder had during the first sprayingoperation, thereby also resulting in a change in the degree of mattnessobtained.

There are also other systems that are intended to obtain matt coatingsin which two polymers of different nature or reactivity are used, aswell as one or more crosslinking agents, so as to induce two distinctcrosslinking mechanisms, or two very different reaction rates. In thesesystems, the powder is prepared in a single step, unlike the processdescribed in U.S. Pat. No. 3,842,035. It is then possible to produce, ina single extrusion operation, a powder paint or varnish capable offorming coatings of very low gloss.

By way of example, Japanese Patent Application 154,771/88 describes aresin composition for a matt powder paint comprising a blend of abranched polyester containing hydroxyl groups having a high hydroxylnumber and of another polyester containing hydroxyl groups having alower hydroxyl number, in defined amounts, and a blocked isocyanate ascrosslinking agent. This composition provides a matt coating having goodmechanical properties and good weatherability. International PatentApplication WO 92/01756 describes powder coating compositions comprisinga blend containing together (1) a semicrystalline polyester containinghydroxyl groups having a hydroxyl number of 20 to 100, (2) an amorphouspolyester containing hydroxyl groups having a hydroxyl number of 20 to120, (3) an acrylic polymer containing hydroxyl groups and (4) a blockedpolyisocyanate as crosslinking agent. The semicrystalline polyestersused are those whose acid constituent contains from 85 to 95 mol % ofterephthalic acid and from 5 to 15 mol % of 1,4-cyclohexane-dicarboxylicacid and whose alcohol constituent contains an aliphatic diol with alinear chain.

Using these compositions, low-gloss coatings (the gloss, measured at anangle of 60°, according to the ASTM D 523 standard, does not exceed 35%)having good mechanical properties and good pencil hardness are obtained.European Patent Application 366,608 describes powder paints, obtained byextrusion, which provide matt coatings, but these paints contain twocross-linking agents. These powder paints contain an epoxy resin,particularly the diglycidylether of bisphenol A, a polycarboxylic acid,such as, for example, 2,2,5,5-tetra(β-carboxyethyl)-cyclopentanone asthe first crosslinking agent, and a saturated polyester terminated bycarboxyl groups, tolylbiguanide or dicyandiamide as the secondcrosslinking agent.

Finally, the use of active compounds in two different reaction systemsfor producing matt coatings is described, for example, in EuropeanPatent 104,424. That patent proposes the preparation of a powder byusing only a single extrusion. This powder contains, as binder, both aresin containing hydroxyl groups, such as a polyester containinghydroxyl groups, and a polyepoxidized compound, such as triglycidylisocyanurate, and it contains a special crosslinking agent which, in itsmolecule, includes both carboxyl groups (in order to react with theepoxidized compound) and blocked isocyanate groups (in order to reactwith the resin containing hydroxyl groups).

The main disadvantage of powder paints and varnishes obtained from anextrusion described in the abovementioned patents is that the propertiesof the powders obtained are very sensitive to variations in theextrusion conditions, such as the extrusion temperature, the sheargradient, etc., and, since it is not easy to control these conditionsprecisely, it is difficult to always produce coatings having the samedegree of mattness from a well-defined composition. In particular, theproblem is to find a composition which, under the usual extrusionconditions, produces a low-gloss coating in a completely reliable andreproducible manner.

European Patent 551,064 proposes to solve this problem by usingthermosetting powder coating compositions which contain, as binder, ablend of a linear polyester containing carboxyl groups having an acidnumber of between 20 and 50 mg of KOH per gramme and of an acryliccopolymer containing glycidyl groups obtained from 5 to 30% by weight ofglycidyl acrylate or methacrylate and from 70 to 95% by weight of methylmethacrylate. These powder compositions make it possible to obtaincoatings of good quality having a very low gloss. This is because thegloss, measured at an angle of 60°, according to the ASTM D 523standard, is always less than 15%. Furthermore, these matt coatings havea smooth surface without any defects, good adhesion to metal surfacesand excellent weatherability. In addition, the properties of thesepowders are only slightly sensitive to variations in the extrusionconditions or are insensitive to them, which means that a definedcomposition practically always produces a matt coating havingsubstantially the same degree of mattness.

However, it has turned out that the matt coatings obtained from thesecompositions do not withstand mechanical deformations, given that themechanical properties of these coatings are insufficient, in particularthe direct impact strength and reverse impact strength.

In conclusion, it may be seen that the various powder compositionsproposed hitherto for obtaining low-gloss coatings all still have acertain number of drawbacks.

There therefore still remains a need to have available thermosettingpowder compositions capable of producing low-gloss coatings that do nothave the defects of the compositions of the prior art.

According to the present invention, it has been surprisingly discoveredthat using, as binder, a blend of an amorphous polyester containingcarboxyl groups, rich in isophthalic acid, and of a semicrystallinepolyester containing carboxyl groups which is prepared more particularlyfrom a saturated aliphatic dicarboxylic acid with a linear chain andfrom a saturated aliphatic diol with a linear or cyclic chain, and of anappropriate crosslinking agent, it is possible to obtain thermosettingpowder compositions which are capable of producing low-gloss coatings ina completely reliable and reproducible manner and which have goodmechanical properties and excellent weatherability.

The subject of the present invention is therefore thermosetting powdercoating compositions comprising, as binder, a blend of an amorphouspolyester, of a semicrystalline polyester and of a crosslinking agent,which are characterized in that the binder comprises:

(a) an amorphous polyester containing carboxyl groups, rich inisophthalic acid, prepared from an acid constituent comprising from 55to 100 mol % of isophthalic acid, from 0 to 45 mol % of at least onedicarboxylic acid other than isophthalic acid and from 0 to 10 mol % ofa polycarboxylic acid containing at least 3 carboxyl groups and from analcohol constituent comprising from 60 to 100 mol % of neopentylglycol,from 0 to 40 mol % of at least one dihydroxylated compound other thanneopentyl-glycol and from 0 to 10 mol % of a polyhydroxylated compoundcontaining at least 3 hydroxyl groups, the said amorphous polyesterhaving a glass transition temperature T_(g)) of at least 50° C. and anacid number of 15 to 100 mg of KOH/g;

(b) a semicrystalline polyester containing carboxyl groups preparedeither

(b1) from a saturated aliphatic dicarboxylic acid with a linear chainhaving from 4 to 16 carbon atoms and from a saturated aliphatic diolwith a linear chain having from 2 to 16 carbon atoms and optionally froma poly-carboxylic acid containing at least 3 carboxyl groups or from apolyol containing at least 3 hydroxyl groups, or

(b2) from 40 to 100 mol % of a saturated aliphatic dicarboxylic acidwith a linear chain having from 10 to 16 carbon atoms and from 0 to 60mol % of an aliphatic dicarboxylic acid with a linear chain having from4 to 9 carbon atoms, calculated with respect to the total of thedicarboxylic acids, from a cycloaliphatic diol having from 3 to 16carbon atoms and optionally from a polycarboxylic acid having at least 3carboxyl groups or from a polyol having at least 3 hydroxyl groups,

the said semicrystalline polyesters having a melting point (T_(m)) of atleast 40° C. and an acid number of 5 to 50 mg of KOH/g; and

(c) a crosslinking agent.

The amorphous polyesters containing carboxyl groups, rich in isophthalicacid, that are used in the compositions according to the invention arepolyesters having a high isophthalic acid content commonly used inpowder paint and varnish formulations for the production of coatingswhich exhibit excellent outdoor exposure performance.

According to the invention, the acid constituent of these amorphouspolyesters rich in isophthalic acid must contain at least 55 mol % ofisophthalic acid and it may even consist completely of isophthalic acid(100 mol %).

The acid constituent of these amorphous polyesters may furthermorecontain up to 45 mol % of an aromatic, aliphatic or cycloaliphaticdicarboxylic acid, other than isophthalic acid, such as terephthalicacid, phthalic acid, succinic acid, glutaric acid, adipic acid, pimelicacid, suberic acid, azelaic acid, sebacic acid, fumaric acid, maleicacid, 1,3-cyclohexanedicarboxylic acid, 1,4-cyclohexanedicarboxylic acidand mixtures of these compounds and up to 10 mol % of a polycarboxylicacid containing at least 3 carboxyl groups, such as trimellitic acid orpyromellitic acid. These acids may be used in the form of the free acidor, if required, in the form of the anhydride, or else in the form of anester with a lower aliphatic alcohol.

The alcohol constituent of these amorphous polyesters must contain atleast 60 mol % of neopentylglycol and it may even consist entirely ofneopentylglycol (100 mol %). The alcohol constituent of these amorphouspolyesters may furthermore contain up to 40 mol % of an aliphatic,cycloaliphatic or aromatic dihydroxylated compound, other thanneopentylglycol, such as ethylene glycol, propylene glycol,1,4-butanediol, 1,6-hexanediol, 2-methyl-1,3-propanediol,2-butyl-2-ethyl-1,3-propanediol, 1,4-cyclohexanedimethanol, hydrogenatedbisphenol A, neopentylglycol hydroxypivalate and mixtures of thesecompounds and up to 10 mol % of a polyhydroxylated compound containingat least 3 hydroxyl groups, such as trimethylolpropane,di-trimethylolpropane, pentaerythritol and mixtures thereof.

The amorphous polyesters containing carboxyl groups, rich in isophthalicacid, which may be used according to the invention have an acid numberof 15 to 100 mg of KOH per gramme, preferably 30 to 70 mg of KOH pergramme, and have a glass transition temperature (T_(g)) which is atleast 50° C. so that the polyesters remain solid at the storagetemperature (20 to 50° C.), and which preferably varies from 50 to 80°C. The number-average molecular weight ({overscore (M)}_(n)) of theseamorphous polyesters is between 1100 and 11,500 and preferably between1600 and 8500. The melt viscosity (measured using a cone-and-plateviscometer according to the ASTM D 4287-88 standard) of these amorphouspolyesters may vary from 100 to 15,000 mPa·s at 200° C.

The semicrystalline polyesters containing carboxyl groups used in thecompositions according to the invention are semicrystalline polyestershaving a particular chemical constitution. They may be chosen from twotypes of polyesters, (b1) and (b2). The polyesters (b1) are prepared bypolyesterification of a saturated aliphatic dicarboxylic acid with alinear chain having from 4 to 16 carbon atoms with a saturated aliphaticdiol having from 2 to 16 carbon atoms. Preferably, these polyesters arelinear, but branched polyesters may also be used in which the branchingis introduced by means of a polycarboxylic acid containing at least 3carboxyl groups, such as trimellitic acid or pyromellitic acid, or of apolyol such as trimethylol-propane, di-trimethylolpropane andpentaerythritol, these compounds being used in an amount ranging from 0to 10 mol % with respect to all the monomers together.

Examples of saturated aliphatic dicarboxylic acids with a linear chainwhich may be used are succinic acid, glutaric acid, pimelic acid,suberic acid, azeleic acid, sebacic acid and 1,12-dodecanedioic acid.These acids may be used as a mixture, but they are preferably used bythemselves. 1,12-Dodecanedioic acid is the preferred acid. Examples ofsaturated aliphatic diols with a linear chain which may be used areethylene glycol, 1,3-propanediol, 1,4-butanediol, 1,5-pentanediol,1,6-hexanediol, 1,7-heptanediol, 1,8-octanediol, 1,9-nonanediol,1,10-decanediol, 1,12-dodecanediol, 1,14-tetradecanediol and1,16-hexadecanediol. These diols may be used as a mixture, but they arepreferably used by themselves. It is preferred to use 1,6-hexanediol.

The semicrystalline polyesters (b2) are prepared from 40 to 100 mol % ofa saturated aliphatic dicarboxylic acid with a linear chain having from10 to 16 carbon atoms and from 0 to 60 mol % of a saturated aliphaticdicarboxylic acid with a linear chain having from 4 to 9 carbon atoms,calculated with respect to all of these dicarboxylic acids together. Itis also possible to use from 40 to 95 mol % of the acid having from 10to 16 carbon atoms and from 5 to 60 mol % of the acid having from 4 to 9carbon atoms. The alcohol constituent of these polyesters is acycloaliphatic diol having from 3 to 16 carbon atoms. Preferably, thesepolyesters are linear but it is also possible to use branched polyestersin which the branching is introduced by means of a polycarboxylic acidcontaining at least 3 carboxyl groups, such as trimellitic acid orpyromellitic acid, or of a polyol such as trimethylol-propane,di-trimethylolpropane and pentaetythritol. These compounds having atleast 3 functions are used in an amount ranging from 0 to 10 mol % withrespect to all of the monomers involved in the polyester composition.

Examples of saturated aliphatic dicarboxylic acids with a linear chainhaving from 10 to 16 carbon atoms are 1,10-decanedioic acid,1,11-undecanedioic acid, 1,12-dodecanedioic acid, 1,13-tridecanedioicacid, 1,14-tetradecanedioic acid, 1,15-pentadecanedioic acid and1,16-hexadecanedioic acid. Preferably, 1,12-dodecanedioic acid is used,by itself or in a mixture.

Examples of saturated aliphatic dicarboxylic acids with a linear chainhaving from 4 to 9 carbon atoms which may be used are succinic acid,glutaric acid, pimelic acid, suberic acid and azelaic acid. These acidsmay be used in a mixture but they are preferably used by themselves.Examples of cycloaliphatic diols having from 3 to 16 carbon atoms whichmay be used are 1,4-cyclohexanediol, 1,4-cyclohexanedimethanol,hydrogenated bisphenol A, 2,2,4,4-tetramethyl-1,3-cyclobutanediol and4,8-bis (hydroxymethyl) tricyclo [5.2.1.0 ^(2,6)] decane.

These diols may be used in a mixture, but they are preferably used bythemselves. It is preferred to use 1,4-cyclohexanediol orcyclohexanedimethanol.

The semicrystalline polyesters containing carboxyl groups which may beused according to the invention have an acid number of 5 to 50 mg of KOHper gramme, preferably 5 to 30 mg of KOH per gramme; preferably, thesepolyesters have a hydroxyl number which does not exceed 5 mg of KOH pergramme.

These semicrystalline polyesters are solid substances at ambienttemperature, characterized by a melting point (T_(m)) of between 40 and90° C.

The number-average molecular weight ({overscore (M)}_(n)) of thesesemicrystalline polyesters is between 2200 and 25,000 and preferablybetween 2800 and 11,220. The melt viscosity (measured using acone-and-plate viscometer according to the ASTM D 4287-88 standard) ofthese semicrystalline polyesters may vary from 50 mPa·s at 100° C to10,000 mPa·s at 150° C.

It has been surprisingly found that only the thermosetting powdercompositions containing both an amorphous polyester rich in isophthalicacid and a particular semicrystalline polyester prepared bypolyesterification of a saturated aliphatic dicarboxylic acid having alinear chain with a saturated aliphatic diol having a linear orcycloaliphatic chain, such as those described in detail above, arecapable of providing low-gloss or matt coatings having good mechanicalproperties and excellent weatherability. This is because it has beenobserved that powders which have an identical composition but in whichthe amorphous polyester rich in isophthalic acid is replaced by anamorphous polyester rich in terephthalic acid, of the type of thosedescribed and used in U.S. Pat. No. 5,373,084 give, after curing, acoating having a very high gloss (Comparative Example I). Likewise, ithas been observed that powders which have an identical composition butin which the particular semicrystalline polyester according to theinvention is replaced by a semicrystalline polyester that is notaccording to the invention, of the type of those described and used inEuropean Patent 521,992 and essentially based on terephthalic acid and1,6-hexanediol also give, after curing, a coating having a very highgloss (Comparative Examples II and III).

The amorphous polyesters rich in isophthalic acid and thesemicrystalline polyesters used according to the invention are preparedby the conventional methods of synthesizing polyesters, using an excessof acid with respect to the alcohol so as to obtain a polyestercontaining carboxyl groups having the desired acid number.

The amorphous polyesters containing carboxyl groups, rich in isophthalicacid, may be prepared by one-stage or two-stage methods of synthesis. Inthe latter case, in the first stage, a polyester containing hydroxylgroups is prepared from, on the one hand, isophthalic acid and,optionally, one or more other polycarboxylic acids other thanisophthalic acid (or their functional derivatives) and from, on theother hand, an excess of neopentylglycol and, optionally, one or moreother dihydroxylated and/or polyhydroxylated compounds and, in thesecond stage, the polyester containing hydroxyl groups thus obtained isesterified with an appropriate dicarboxylic acid, preferably isophthalicacid, in order to obtain an amorphous polyester containing carboxylgroups, rich in isophthalic acid.

The semicrystalline polyesters containing carboxyl groups are generallyprepared using a one-stage process from an appropriate aliphatic orcycloaliphatic diol and from an excess of the appropriate aliphaticdicarboxylic acid, optionally incorporating a polycarboxylic acid or apolyol into the reaction mixture if it is desired to obtain a branchedsemicrystalline polyester.

The synthesis of these polyesters is generally carried out in aconventional reactor fitted with a stirrer, with an influx of inert gas(for example, nitrogen), with a distillation column connected to acondenser and with a thermometer attached to a temperature controller.

The esterification conditions are conventional, namely that a standardesterification catalyst may be used, either a tin derivative, such asdibutyltin dilaurate, dibutyltin oxide or n-butyltin trioctanoate, or atitanium derivative, such as tetrabutyl titanate, in an amount rangingfrom 0.01 to 1% by weight of the reactants. To this may optionally beadded an antioxidant of the phenolic type, such as Irganox 1010 (sold byCiba-Geigy), by itself or in a mixture with a stabilizer, such as, forexample tributyl phosphite, in an amount ranging from 0.01 to 1% byweight of the reactants.

The polyesterification is generally carried out at a temperature whichis gradually increased from approximately 130° C. to approximately 180to 250° C., firstly under normal pressure and then under reducedpressure, maintaining this temperature until the formation of apolyester having the desired hydroxyl number and/or acid number. When atwo-stage process is used to prepare the amorphous polyesters rich inisophthalic acid, the reaction mixture containing the polyestercontaining hydroxyl groups that was obtained in the first stage is leftto cool down to 200° C., the desired amount of dicarboxylic acid isadded, the temperature is raised to 230-240° C. and this temperature ismaintained firstly under normal pressure and then under reducedpressure, until the formation of an amorphous polyester containingcarboxyl groups having the desired acid number.

The degree of esterification is monitored by determining the amount ofwater formed during the reaction and the properties of the polyesterobtained, for example the acid number, the hydroxyl number, themolecular weight, the glass transition temperature (T_(g)), the meltingpoint T_(m)) and the melt viscosity.

At the end of the synthesis, when the polyester is still in the moltenstate, a crosslinking catalyst, known per se, may optionally be added toit in an amount ranging from 0.01 to 1.5% by weight of the polyester.These catalysts may be of the amine type, such as 2-phenylimidazoline,of the phosphine type, such as triphenylphosphine, or ammonium orphosphonium salts, such as tetrapropylammonium chloride,tetrabutylammonium bromide, benzyltriphenylphosphonium chloride orethyltriphenylphosphonium bromide. Next, the polyester is removed fromthe reactor and cast as a thick layer, left to cool and ground intoparticles having an average size ranging from a fraction of an mm toseveral mm.

The amorphous polyester containing carboxyl groups, rich in isophthalicacid, the semicrystalline polyester containing carboxyl groups and thecrosslinking agent together form the basic binder for the thermosettingpowder coating compositions according to the invention.

In the thermosetting powder coating compositions according to theinvention, the amount of amorphous polyester rich in isophthalic acidgenerally represents approximately 60 to 87% by weight and the amount ofsemicrystalline polyester approximately 13 to 40% by weight calculatedwith respect to the total weight of the amorphous polyester and of thesemicrystalline polyester. However, it has been observed that the gloss,measured at an angle of 60° C. according to the ASTM D 523 standard, ofthe coatings obtained decreases when the amount of amorphous polyesterdecreases and the amount of semicrystalline polyester increases. This iswhy, in the compositions according to the invention, the amount ofamorphous polyester rich in isophthalic acid preferably represents 60 to82% by weight and the amount of semicrystalline polyester preferablyrepresents 18 to 40% by weight calculated with respect to the totalweight of the amorphous polyester and of the semi-crystalline polyester.

The crosslinking agents which may be used in the compositions accordingto the invention are all organic compounds having functional groupscapable of reacting with the carboxyl groups of the polyesters in orderto crosslink the binder. Typical crosslinking agents are, for example,polyepoxidized compounds and β-hydroxyalkylamides. Particularlypreferred cross-linking agents are triglycidyl isocyanurate (ARALDITE PT810 sold by Ciba-Geigy), the 75/25 mixture of diglycidyl terephthalateand of triglycidyl trimellitate (ARALDITE PT 910 sold by Ciba-Geigy),acrylic copolymers containing glycidyl groups, such as GMA 252 sold byEstron, and bis(N,N-dihydroxyethyl)-adipamide (PRIMID XL 552 sold byEMS).

In the thermosetting powder coating compositions according to theinvention, the ratio of the amount of amorphous polyester and ofsemicrystalline polyester containing carboxyl groups, on the one hand,to the amount of crosslinking agent, on the other hand, is such thatthere are from 0.5 to 1.5 equivalents of carboxyl groups per equivalentof functional groups (for example, epoxy groups) in the crosslinkingagent.

It is preferred to use the crosslinking agent in an amount ofapproximately 4 to 25% by weight calculated with respect to the totalweight of the binder.

The powder coating compositions according to the invention thereforegenerally contain, as binder, (a) from 45 to 83% by weight of amorphouspolyester rich in isophthalic acid, (b) from 13 to 30% by weight ofsemicrystalline polyester and (c) from 4 to 25% by weight ofcrosslinking agent, and preferably (a) from 45 to 78% by weight ofamorphous polyester rich in isophthalic acid, (b) from 18 to 30% byweight of semicrystalline polyester and (c) from 4 to 25% by weight ofcrosslinking agent.

The present invention also relates to the use of the thermosettingpowder coating compositions according to the invention for thepreparation of powder paints and varnishes that provide low-glosscoatings, preferably matt coatings, as well as to the powder paints andvarnishes obtained using these compositions.

The powder paints and varnishes according to the invention may beprepared by mixing in a homogeneous manner the amorphous polyester richin isophthalic acid, the semicrystalline polyester and the crosslinkingagent with the various auxiliary substances conventionally used for themanufacture of powder paints and varnishes.

This homogenization is carried out, for example, by firstly dryblending, at ambient temperature, the amorphous polyester, thesemi-crystalline polyester, the crosslinking agent and the variousauxiliary substances in a mixer, for example a drum mixer, and by thenpassing the blend thus obtained through an extruder, for example asingle-screw extruder of the Buss-Ko-Kneter type or a twin-screwextruder of the PRISM or A.P.V. type in order to produce melthomogenization therein at a temperature lying within the range 80 to150° C. The extrudate is then left to cool, ground and screened in orderto obtain a powder whose particle size is between 10 and 150micrometres.

If desired, the amorphous polyester rich in isophthalic acid and thesemicrystalline polyester may be melt blended beforehand in thesynthesis reactor or in an extruder, such as a Betol BTS 40 extruder,before they are added in the form of a blend to the other ingredients ofthe composition for the dry blending at ambient temperature. However, itis more convenient to add the two polyesters separately to the otheringredients of the powder composition.

The auxiliary substances which may be added to the thermosetting powdercoating compositions are, for example, pigments and colorants, such astitanium dioxide, iron oxides, zinc oxide, etc., metal hydroxides, metalpowders, sulphides, sulphates, carbonates, silicates, such as ammoniumsilicate for example, carbon black, talc, kaolins, barytes, iron blues,lead blues, organic reds, organic browns, etc., flow-regulating agents,such as RESIFLOW PV5 (from Worlee), MODAFLOW (from Monsanto) or ACRONAL4F (from BASF), and air-release agents, such as benzoin, etc. Theseauxiliary substances are used in the usual amounts, it being understoodthat if the thermosetting compositions according to the invention areused as varnishes the addition of opacifying substances will be omitted.It is also possible to add compounds that absorb ultraviolet radiation,such as TINUVIN 900 from Ciba-Geigy and sterically hindered amine-basedlight stabilizers, such as TINUVIN 144 from Ciba-Geigy.

The subject of the present invention is also a process for obtaining alow-gloss coating on an article, in which process a thermosetting powdercoating composition according to the invention, as described above, isapplied to the said article and the article thus coated is cured at ahigh temperature for a time long enough to completely crosslink thecoating.

The thermosetting powder coating compositions may be applied to articlesof various shapes and sizes, in particular to articles made of glass, ofceramic and of metal, such as steel and aluminium, using techniquesknown per se for depositing powders, that is to say by using a spray gunin an electrostatic field in which the powder is charged under a voltageof 30 to 100 kV by a high-voltage direct current, or by using atriboelectric spray gun in which the powder is charged by friction, orelse by the well-known technique of fluidized-bed deposition.

After they have been applied to the article in question, the depositedpowders are oven-cured at a temperature of between 140 and 200° C. for atime which may be as long as 30 minutes in order to melt the powderparticles, to form a homogeneous skin, which spreads perfectly over thesubstrate, and finally to achieve complete crosslinking and curing ofthe coating.

The thermosetting powder coating compositions according to the presentinvention allow coatings to be obtained which have a low gloss. This isbecause the gloss, measured at an angle of 60° C. according to the ASTMD 523 standard, of these coatings always has a value of less than 50%.

In addition, as has already been explained above, it is possible toobtain coatings having different levels of gloss depending on the amountof amorphous polyester and on the amount of semi-crystalline polyesterthat are used in these compositions.

By virtue of the present invention, it is therefore possible, aftercuring, to obtain coatings having the desired level of gloss simply bychoosing a powder composition which contains the required proportions ofamorphous polyester and of semi-crystalline polyester.

However, account should be taken of the fact that the nature of thecrosslinking agent is also important with respect to the level of glossof the coatings obtained after curing (see Example 11 below). A fewpreliminary tests will make it possible to easily determine thecomposition that is suitable for obtaining a coating which has thedesired level of gloss.

As shown in greater detail in the examples which follow, it is thereforepossible, by virtue of the present invention, to prepare powder paintsand varnishes capable of providing coatings having the level of glossrequired by the application envisaged, that is to say satin orsemi-gloss coatings, the gloss of which, measured at an angle of 60° C.according to the ASTM D 523 standard, is less than 50%, or mattcoatings, the gloss of which, measured under the same conditions, isless than 35%.

Furthermore, the thermosetting powder compositions according to thepresent invention are capable of producing low-gloss coatings having acombination of other advantageous properties, inter alia a smooth anduniform appearance, without any surface defects and free of “orangepeel”, good mechanical properties, and excellent weatherability and UVresistance. The formation of a satin or matt finish is therefore notaccompanied by a deterioration in the other properties of the coatings,as is often the case with powder compositions of the prior art that arecapable of producing low-gloss coatings.

Finally, another important advantage of the thermosetting powder coatingcompositions according to the present invention over the powdercompositions of the prior art resides in the fact that their propertiesare only slightly sensitive, or are insensitive, to variations in theextrusion conditions and that these compositions therefore make itpossible to continually produce low-gloss, satin or matt coatings havingsubstantially the same degree of gloss or of mattness.

The examples which follow illustrate the invention without limiting it.In these examples, certain characteristic values have been determinedaccording to the methods described below:

gloss: this is expressed as the intensity of the reflected light, in percent, with respect to the intensity of the light incident at an angle of60° and measured according to the ASTM D 523 standard;

impact strength: this is measured by means of a Gardner apparatusaccording to the ASTM D 2794 standard. Cold-rolled steel panels providedwith a cured coating are subjected to impacts of increasing intensity onthe coated side (direct impact) and on the uncoated side (reverseimpact). The highest impact that does not cause the coating to crack ismeasured in kg.cm;

pencil hardness: this is determined by means of a WOLFF WILBRON hardnesstester (ASTM D 3363 standard); the hardness measured is that of thehardest pencil that does not cause a notch in the coating according to ascale which goes from 6B (the softest pencil) to 6H (the hardestpencil);

QUV accelerated ageing test: chromium-plated aluminium panels providedwith a cured coating to be tested are placed in a “QUV Panel” testapparatus from the company Q-Panel Co. (Cleveland, USA), and subjectedto several cycles of exposure to UV lamps and to moisture, at varioustemperatures. Among the various cycles of this type that are describedin the ASTM G 53-88 standard, the coatings in the present case weresubjected to a cycle of 8 hours' exposure to a fluorescent UVA lamp (340nm wavelength and 0.77 W/m²/nm intensity) simulating the harmful effectsof sunlight, at 60° C., and of 4 hours of water vapour condensation,with the lamp off, at 40° C.; the change in gloss, measured at an angleof 60°, according to the ASTM D 523 standard, is noted. After 3000hours' exposure to this test, the gloss retention was determined fromthe equation:${\% \quad {retention}} = {\frac{60{^\circ}\quad {gloss}\quad {after}\quad 3000\quad {hours}^{\prime}\quad {exposure}}{60{^\circ}\quad {gloss}\quad {at}\quad {the}\quad {start}\quad {of}\quad {the}\quad {experiment}} \times 100\%}$

and also the colour change delta E, calculated according to the ASTM D2244 standard;

the acid number and the hydroxyl number were determined by titrationaccording to the DIN 53402 and DIN 53240 standards and expressed in mgof KOH per gramme of polyester;

the glass transition temperature (T_(g)) and the melting point (T_(m))were determined by differential scanning calorimetry (DSC) at a scanrate of 20° C. per minute;

the number-average molecular weight ({overscore (M)}_(n)) of thepolyesters was determined from the equation:${\overset{\_}{M}}_{n} = \frac{{{functionality} \times 56},100}{N_{A}}$

where N_(A) is the acid number expressed in mg of KOH per gramme ofpolyester; and

the melt viscosity of the polyesters, expressed in mPa·s, was measuredby means of an ICI cone-and-plate viscometer according to the ASTM D4287-88 standard; it is also called the “ICI viscosity” and was measuredat the temperatures indicated in the examples.

Unless otherwise indicated, the parts mentioned in the examples areparts by weight.

EXAMPLE 1 Synthesis of Amorphous Polyesters

a) One-Stage Synthesis

A mixture a 399.6 parts (3.84 mol) of neopentyl-glycol and 22.2 parts(0.16 mol) of trimethylol-propane is introduced into a 10 litrefour-necked round-bottomed flask provided with a stirrer, a nitrogeninflux and a distillation column connected to a water-cooled condenserand a thermometer attached to a temperature controller. The mixture isheated, with stirring and under nitrogen, to a temperature ofapproximately 130° C. and added to it are 722.9 parts (4.35 mol) ofisophthalic acid and 2.5 parts of n-butyltin trioctanoate asesterification catalyst. The temperature of the reaction mixture is thengradually raised to 230° C. Water starts to distil from the reactorabove 180° C. When the distillation of water at atmospheric pressure iscompleted, a vacuum of 50 mm Hg is gradually created. The temperature ofthe reaction mixture is maintained for 3 hours at 230° C. and at apressure of 50 mm Hg.

Finally, the polyester obtained is left to cool to 180° C. and removedfrom the reactor. The polyester containing carboxyl groups thus obtainedhas the following characteristics:

Acid number: 32 mg of KOH/g

Hydroxyl number: 2 mg of KOH/g

ICI viscosity (at 200° C.): 8000 mPa·s

Glass transition temperature (T_(g)): 59° C. (DSC at 20° C./minute)

Number-average molecular weight ({overscore (M)}_(n)): 5423(theoretical)

b) Two-Stage Synthesis

1st Stage:

A mixture of 423.5 parts (4.07 mol) of neopentylglycol and 22.2 parts(0.16 mol) of trimethylolpropane is introduced into a reactor asdescribed at a) above. The mixture is heated with stirring and undernitrogen to a temperature of approximately 130° C. and added to it are121.8 parts (0.733 mol) of terephthalic acid, 487.1 parts (2.93 mol) ofisophthalic acid and 2.3 parts of n-butyltin trioctanoate asesterification catalyst.

The reaction is continued at 220° C. and under atmospheric pressureuntil approximately 95% of the theoretical amount of water is distilled.A transparent polyester containing hydroxyl groups is obtained which hasthe following characteristics:

Hydroxyl number: 59 mg of KOH/g

Acid number: 12 mg of KOH/g

ICI viscosity (at 175° C.): 2200 mPa·s

2nd Stage:

The polyester obtained at the first stage is left too cool to 200° C.and added to it are 110.9 parts (0.67 mol) of isophthalic acid. Next,the mixture is gradually heated to 230° C. The mixture is maintained atthis temperature for 2 hours and, when the reaction mixture becomesclear, a vacuum of 50 mm Hg is gradually created. The reaction iscontinued for 3 hours at 230° C. and under a pressure of 50 mm Hg.

The polyester containing carboxyl groups thus obtained has the followingproperties:

Acid number: 31 mg of KOH/g

Hydroxyl number: 3 mg of KOH/g

ICI viscosity (at 200° C.): 6600 mPa·s

Glass transition temperature (T_(g)): 57° C. (DSC at 20° C./minute)

Number-average molecular weight ({overscore (M)}_(n)): 5423(theoretical)

c) Yet another amorphous polyester containing carboxyl groups, rich inisophthalic acid, is prepared using the two-stage operating methoddescribed in b) above.

In the first stage, a polyester containing hydroxyl groups is preparedin the same way as in b) above from 423.5 parts (4.07 mol) ofneopentyl-glycol, 304.5 parts (1.83 mol) of terephthalic acid, 304.5parts (1.83 mol) of isophthalic acid and 2.3 parts of n-butyltintrioctanoate as esterification catalyst. The reaction is carried outunder atmospheric pressure at 230° C. (instead of 220° C.).

The transparent polyester containing hydroxyl groups has the followingcharacteristics:

Hydroxyl number: 57 mg of KOH/g

Acid number: 13 mg of KOH/g

ICI viscosity (at 175° C.): 1200 mPa·s

In the second stage, a polyester containing carboxyl groups is preparedin the same way as in b) above: 110.9 parts (0.67 mol) of isophthalicacid are added as in b) above.

The polyester containing carboxyl groups thus obtained has the followingcharacteristics:

Acid number: 32 mg of KOH/g

Hydroxyl number: 2.1 mg of KOH/g

ICI viscosity (at 200° C.): 3000 mPa·s Glass transition temperature(T_(g)): 53° C. (DSC at 20° C./minute)

Number-average molecular weight ({overscore (M)}_(n)): 3740(theoretical)

d) By way of comparison, an amorphous polyester containing carboxylgroups, rich in terephthalic acid, is also prepared according to thetwo-stage operating method described in b) above; it contains a minoramount of isophthalic acid (16.2 mol %) and a major amount ofterephthalic acid (83.8 mol %).

In the first stage, a polyester containing hydroxyl groups is preparedin the same manner as in b) above from 417.8 parts (4.02 mol) ofneopentylglycol, 600.7 parts (3.62 mol) of terephthalic acid and 2.2parts of n-butyltin trioctanoate as esterification catalyst. Thereaction is carried out under atmospheric pressure at 240° C. (insteadof 220° C.).

The transparent polyester containing hydroxyl groups has the followingcharacteristics:

Hydroxyl number: 61 mg of KOH/g

Acid number: 8 mg of KOH/g

ICI viscosity (at 200° C.): 1200 mPa·s

In the second stage, a polyester containing carboxyl groups is preparedin the same way as in b) above, but 117.1 parts (0.7 mol) of isophthalicacid are added (instead of 110.9 parts); the mixture is gradually heatedto 240° C. (instead of 230° C.) and it is maintained at this temperaturefor 3 hours. When the reaction mixture becomes clear, a vacuum of 50 mmof Hg is gradually created and the reaction continued for 4 hours at240° C. and under a pressure of 50 mm Hg. The polyester containingcarboxyl groups thus obtained has the following characteristics:

Acid number: 33 mg of KOH/g

Hydroxyl number: 3 mg of KOH/g

ICI viscosity (at 200° C.): 4700 mPa·s

Glass transition tempera-ture (T_(g)): 57° C.

(DSC at 20°C./minute) Number-average molecular weight ({overscore(M)}_(n)): 3740 (theoretical)

The polyester obtained is left to cool to 200° C. and added to it are1.3 parts of ethyltriphenyl-phosphonium bromide (crosslinking catalyst).After stirring the mixture for one hour, the polyester is removed fromthe reactor.

e) Another amorphous polyester containing carboxyl groups, rich inisophthalic acid, is prepared using the one-stage operating methoddescribed in a) above from 424.9 parts of neopentylglycol, 722.2 partsof isophthalic acid and 2.3 parts of n-butyltin trioctanoate asesterification catalyst. The polyester containing carboxyl groups thusobtained has the following characteristics:

Acid number: 31.5 mg of KOH/g

ICI viscosity (at 200° C.): 2900 mPa·s

Glass transition tempera-ture (T_(g)): 52° C.

(DSC at 20° C./minute)

EXAMPLE 2 Synthesis of Semicrystalline Polyesters.

a) A mixture of 739.9 parts (3.21 mol) of 1,12-dodecanedioic acid, 369.2parts (3.12 mol) of 1,6-hexanediol and 2.5 parts of n-butyltintrioctanoate as esterification catalyst are introduced into a reactor asdescribed in Example 1.

The mixture is heated, with stirring and under nitrogen, to atemperature of approximately 140° C., at which temperature the waterstarts to distil from the reactor. The temperature of the reactionmixture is then gradually raised to 225° C. When the distillation ofwater at atmospheric pressure is completed, 1.0 part of tributylphosphite as stabilizer and 1.0 part of n-butyltin trioctanoate areadded and a vacuum of 50 mm Hg is gradually created. The temperature ofthe reaction mixture is maintained for 3 hours at 225° C. and under apressure of 50 mm Hg.

The semicrystalline polyester containing carboxyl groups thus obtainedhas the following characteristics:

Acid number: 11.5 mg of KOH/g

Hydroxyl number: 0.5 mg of KOH/g

ICI viscosity (at 150° C.): 7000 mPa·s

Melting point (T_(m)) (DSC at 20° C./minute): 67° C.

Number-average molecular weight ({overscore (M)}_(n)): 11220(theoretical)

The polyester obtained is left to cool to 160° C. and added to it are 10parts of TINUVIN 144 (light stabilizer) and 20 parts of TINUVIN 900 (UVlight absorber). After stirring the mixture for one hour, the polyesteris removed from the reactor, left to cool to ambient temperature andobtained in the form of a white solid substance.

b) Another semicrystalline polyester is prepared using the operatingmethod described in a) above from 750.7 parts (3.26 mol) of1,12-dodecanedioic acid, 353.7 parts (2.99 mol) of 1,6-hexanediol and2.5 parts of n-butyltin trioctanoate as esterification catalyst.

The semicrystalline polyester containing carboxyl groups thus obtainedhas the following characteristics:

Acid number: 29.5 mg of KOH/g

Hydroxyl number: 0.6 mg of KOH/g

ICI viscosity (at 100° C.): 2000 mPa·s Melting point (T_(m)) (DSC at 20°C./minute): 65° C.

Number-average molecular weight ({overscore (M)}_(n)): 3740(theoretical)

The polyester obtained is left to cool to 160° C. and added to it are 10parts of TINUVIN 144 and 20 parts of TINUVIN 900. After stirring themixture for one hour, the polyester is removed from the reactor, left tocool to ambient temperature and obtained in the form of a white solidsubstance.

c) Another semicrystalline polyester is prepared using the operatingmethod described in a) above from 750.2 parts (3.25 mol) of1,12-dodecanedioic acid, 351.2 parts (2.97 mol) of 1,6-hexanediol, 9.567parts (0.07 mol) of trimethylolpropane and 2.5 parts of n-butyltintrioctanoate as esterification catalyst.

The semicrystalline polyester containing carboxyl groups thus obtainedhas the following characteristics:

Acid number: 21.3 mg of KOH/g

Hydroxyl number: 0.9 mg of KOH/g

ICI viscosity (at 100° C.): 4000 mPa·s Melting point (T_(m)) (DSC at 20°C./minute): 63° C.

Number-average molecular weight ({overscore (M)}_(n)): 7012(theoretical)

The polyester obtained is left to cool to 160° C. and added to it are 10parts of TINUVIN 144 and 20 parts of TINUVIN 900. After stirring themixture for one hour, the polyester is removed from the reactor, left tocool to ambient temperature and obtained in the form of a white solidsubstance.

d) Another semicrystalline polyester is prepared using the operatingmethod described in a) above from 897.4 parts (3.90 mol) of1,12-dodecanedioic acid, 236.4 parts (3.81 mol) of ethylene glycol and2.5 parts of n-butyltin trioctanoate as esterification catalyst.

The semicrystalline polyester containing carboxyl groups thus obtainedhas the following characteristics:

Acid number: 8.0 mg of KOH/g

Hydroxyl number: 2.5 mg of KOH/g

ICI viscosity (at 150° C.): 7200 mPa·s

Melting point (T_(m)) (DSC at 20° C./minute): 76° C.

Number-average molecular weight ({overscore (M)}_(n)): 11220(theoretical)

The polyester obtained is left to cool to 160° C. and added to it are 10parts of TINUVIN 144 and 20 parts of TINUVIN 900. After stirring themixture for one hour, the polyester is removed from the reactor, left tocool to ambient temperature and obtained in the form of a white solidsubstance.

e) By way of comparison, a semicrystalline polyester containing carboxylgroups not according to the invention is also prepared in the followingmanner:

1st stage:

453.4 parts (3.84 mol) of 1,6-hexanediol are introduced into a reactoras described in Example 1. The contents of the reactor are heated up to150° C. in order to melt the substance and added to it are 589.9 parts(3.55 mol) of terephthalic acid and 2.3 parts of n-butyltin trioctanoateas esterification catalyst. The reaction is continued at 235° C. underatmospheric pressure until approximately 95% of the theoretical amountof water is distilled. A polyester containing hydroxyl groups isobtained which has the following characteristics:

Hydroxyl number: 40 mg of KOH/g

Acid number: 5 mg of KOH/g

ICI viscosity (at 175° C.): 800 mPa·s 2nd stage:

The polyester obtained in the first stage is left to cool to 200° C. andadded to it are 91.7 parts (0.55 mol) of isophthalic acid. Next, themixture is gradually heated to 235° C. The mixture is maintained at thistemperature for 2 hours, 1.0 part of tributylphosphite is added asstabilizer and a vacuum of 50 mm Hg is gradually created. Thetemperature of the reaction mixture is maintained for 2 hours at 235° C.under a pressure of 50 mm Hg. The semicrystalline polyester containingcarboxyl groups thus obtained has the following characteristics:

Acid number: 32 mg of KOH/g

Hydroxyl number: 0.5 mg of KOH/g

ICI viscosity (at 150° C.): 7200 mPa·s Melting point (T_(m)) (DSC at 20°C./minute): 130° C.

Number-average molecular weight ({overscore (M)}_(n)): 3740(theoretical)

The polyester obtained is left to cool to 160° C. and added to it are 10parts of TINUVIN 144 and 20 parts of TINUVIN 900. After stirring themixture for one hour, the polyester is removed from the reactor, left tocool to ambient temperature and obtained in the form of a white solidsubstance.

f) By way of comparison, yet another semicrystalline polyester isprepared in exactly the same way as in d) above from 458.24 parts (3.88mol) of 1,6-hexanediol and 589.2 parts (3.55 mol) of terephthalic acid(in the first stage) and from 91.4 parts (0.62 mol) of adipic acid (inthe second stage).

The semicrystalline polyester containing carboxyl groups thus obtainedhas the following characteristics:

Acid number: 34 mg of KOH/g

Hydroxyl number: 3 mg of KOH/g

ICI viscosity (at 200° C.): 700 mPa·s Melting point (T_(m)) (DSC at 20°C./minute): 129° C.

Number-average molecular weight ({overscore (M)}_(n)): 3400(theoretical)

This semicrystalline polyester has the same characteristics as thesemicrystalline polyester described in Example 1a of European Patent521,992.

On completion of the synthesis, the polyester obtained is left to cooland added to it are 10 parts of TINUVIN 144 and 20 parts of TINUVIN 900.After stirring the mixture for one hour, the polyester is removed fromthe reactor, left to cool to ambient temperature and obtained in theform of a white solid substance.

g) Another semicrystalline polyester is prepared using the operatingmethod described in a) above from 757.8 parts of 1,12-dodecanedioicacid, 351.2 parts of 1,4-cyclohexanediol and 2.5 parts of n-butyltintrioctanoate as esterification catalyst. The semicrystalline polyestercontaining carboxyl groups thus obtained has the followingcharacteristics:

Acid number: 31.3 mg of KOH/g

ICI viscosity (at 150° C.): 1500 mPa·s

Melting point (T_(m)) (DSC at 20° C./minute): 65° C.

h) Another semicrystalline polyester is prepared using the operatingmethod described in a) above from 700.1 parts of 1,12-dodecanedioicacid, 399.8 parts of 1,4-cyclohexanedimethanol and 2.5 parts ofn-butyltin trioctanoate as esterification catalyst.

The semicrystalline polyester containing carboxyl groups thus obtainedhas the following characteristics:

Acid number: 33.4 mg of KOH/g

ICI viscosity (at 150° C.): 1500 mPa·s

Melting point (T_(m)) (DSC at 20° C./minute): 47° C.

Another semicrystalline polyester is prepared using the operating methoddescribed in a) above from 709.4 parts of 1,12-dodecanedioic acid,369.74 parts of 1,4-cyclohexanedimethanol and 2.5 parts of n-butyltintrioctanoate as esterification catalyst.

The semicrystalline polyester containing carboxyl groups thus obtainedhas the following characteristics:

Acid number: 33.0 mg of KOH/g

ICI viscosity (at 150° C.): 3400 mPa·s

Melting point (T_(m)) (DSC at 20° C./minute): 45° C.

j) Another semicrystalline polyester is prepared using the operatingmethod described in a) above from 500.7 parts of 1,12-dodecanedioicacid, 166.9 parts of adipic acid, 439.8 parts of1,4-cyclohexanedimethanol and 2.5 parts of n-butyltin trioctanoate asesterification catalyst. The semicrystalline polyester containingcarboxyl groups thus obtained has the following characteristics:

Acid number: 32.4 mg of KOH/g

ICI viscosity (at 150° C.): 1300 mPa·s

Melting point (T_(m)) (DSC at 20° C./minute): 45° C.

EXAMPLES 3 TO 15 AND COMPARATIVE EXAMPLES I, II and III Preparation ofThermosetting Powder Coating Compositions

Sixteen thermosetting powder compositions are prepared in the followingmanner. An amorphous polyester containing carboxyl groups, prepared asdescribed in Example 1, a semicrystalline polyester containing carboxylgroups, prepared as described in Example 2, a crosslinking agent andvarious auxiliary substances conventionally used for the manufacture ofpowder paints are dry blended at ambient temperature. The nature and theamounts of these substances are given in Table I below. The blendobtained is homogenized in a twin-screw extruder (PRISM, 16 mm, 15/1L/D) at an extrusion temperature of 85° C. The extrudate is cooled,crushed and ground in a RETSCH ZM 100 mill (0.5 μm screen) and thenscreened in order to form a powder whose particle size is between 10 and100 micrometres.

In order to determine the properties of the coatings obtained usingthese compositions, the powders obtained are deposited by spraying themwith a Gema-Volstatic PCG 1 electrostatic spray gun on to cold-rolledsteel panels under a voltage of 60 to 100 kV so as to obtain a filmthickness of between 50 and 70 micrometres.

The panels thus coated are then transferred into an air-ventilated oven,where the compositions deposited are cured for 15 minutes at atemperature of 200° C.

The cured coatings obtained all have a smooth and uniform appearance andhave no defects such as craters, pinholes or “orange peel”.

The various compositions and the properties of the coatings obtained aredescribed in Table I below.

TABLE I Examples of white paints Composition (parts by weight) 3 4 5 6 78 9 10 11 12 13 14 15 I (1) II (1) III (1) Amorphous polyester ofExample 1a 423 446.4 441.6 — — 441.6 446.4 474.3 427.3 — — — 512.9 —446.4 446.4 of Example 1b — — — 446.4 — — — — — — — — — — — — of Example1c — — — — 423 — — — — — — — — — — — of Example 1d⁽¹⁾ — — — — — — — — —— — — — 446.4 — — of Example 1e — — — — — — — — — 512.9 512.9 512.9 — —— — Semicrystalline polyester of Example 2a 141 111.6 110.4 111.6 141 —— — 142.5 — — — — 111.6 — — of Example 2b — — — — — — — 83.7 — — — — — —— — of Example 2c — — — — — 110.4 — — — — — — — — — — of Example 2d — —— — — — 111.6 — — — — — — — — — of Example 2e⁽¹⁾ — — — — — — — — — — — —— — 111.6 — of Example 2f⁽¹⁾⁽⁷⁾ — — — — — — — — — — — — — — — 111.6 ofExample 2g — — — — — — — — — 128.5 — — — — — — of Example 2h — — — — — —— — — — 128.5 — — — — — of Example 2i — — — — — — — — — — — 128.5 — — —— of Example 2j — — — — — — — — — — — — 128.5 — — — Crosslinking 36 42 —42 36 — 42 42 — 48.3 48.3 48.3 48.3 42 42 42 agent A⁽²⁾ Crosslinking — —48 — — 48 — — — — — — — — — — agent B⁽³⁾ Crosslinking — — — — — — — — 30— — — — — — — agent C⁽⁴⁾ Titanium dioxide⁽⁵⁾ 300 300 300 300 300 300 300300 300 296 296 296 296 300 300 300 Fix F white 100 100 100 100 100 100100 100 100 — — — — 100 100 100 Flow-regulating agent⁽⁶⁾ 10 10 10 10 1010 10 10 10 9.9 9.9 9.9 9.9 10 10 10 Benzoin 3.5 3.5 3.5 3.5 3.5 3.5 3.53.5 3.5 3.4 3.4 3.4 3.4 3.5 3.5 3.5 BTC⁽⁸⁾ — — — — — — — — — 1.0 1.0 1.01.0 — — — Properties 60° gloss 18 29 23 25 34 29 28 44 49 40 35 49 48 8184 86 Impact strength (kg.cm) direct 200 200 80 140 180 100 160 120 200100 100 200 140 160 40 60 reverse 200 200 100 160 180 120 160 120 200100 80 200 140 160 20 40 Pencil hardness H 2H H H H H H 2H H — — — — H H2H ⁽¹⁾by way of comparison ⁽²⁾triglycidyl isocyanurate (ARALDITE PT 810from Ciba-Geigy) ⁽³⁾75/25 mixture of diglycidyl terephthalate andtriglycidyl trimellitate (ARALDITE PT 910 from Ciba-Geigy)⁽⁴⁾bis(N,N-dihydroxyethyl)adipamide (PRIMID XL-552 from EMS) ⁽⁵⁾KRONOS2310 (Ciba-Geigy) ⁽⁶⁾RESIFLOW PV5 (Worlée Chemie) ⁽⁷⁾Polyester describedin Example 1a of European Patent 521,992 ⁽⁸⁾Benzyltriphenylphosphoniumchloride

It may be seen that only the compositions according to the inventioncontaining both an amorphous polyester rich in isophthalic acid and asemicrystalline polyester prepared from a saturated aliphaticdicarboxylic acid with a linear chain (1,12-dodecanedioic acid) and froma saturated aliphatic diol with a linear chain (1,6-hexanediol orethylene glycol) or from a cycloaliphatic diol (1,4-cyclohexanediol or1,4-cyclohexanedimethanol) give coatings whose gloss is less than 50%.

The composition of Comparative Example I, which contains an amorphouspolyester rich in terephthalic acid, gives a coating which is veryglossy (gloss of 81%).

Likewise, the compositions of Comparative Example II and of Example IIIwhich contain a semi-crystalline polyester not according to theinvention, essentially based on terephthalic acid and 1,6-hexanediol (asin the illustrative embodiments in European Patent 521,992), givecoatings which are very glossy (gloss of 84-86%). Furthermore, thecoatings obtained from these compositions have an impact strength and aflexibility which are inferior.

It may also be seen that, when a polyepoxidized compound is used in thecomposition as crosslinking agent and when the amount of semicrystallinepolyester increases (from 15 to 25% by weight) and the amount ofamorphous polyester decreases (from 85 to 75% by weight with respect tothe total weight of the polyesters), the gloss goes from 44 to 18%(compare Examples 10 and 3).

A coating may therefore be produced which has the desired level ofgloss, either a matt coating (Examples 3 to 9) or a satin or semi-glosscoating (Examples 10 and 11), just by appositely choosing the powdercomposition which provides this level of gloss.

EXAMPLES 16 AND 17 AND COMPARATIVE EXAMPLES IV TO VI Preparation ofThermosetting Powder Coating Compositions.

In these examples and comparative examples, 5 thermosetting powdercompositions are prepared using the operating method described in theabove Examples 3 to 15 for the purpose of obtaining dark brown paintsand of testing the properties of the coatings obtained.

The various compositions and the properties of the coatings obtained aregiven in Table II below.

The cured coatings obtained from these compositions all have a smoothand uniform appearance and have no defects.

TABLE II Examples of dark brown paints Composition (parts by weight) 1617 IV (1) V (1) VI (1) Amorphous polyester of Example 1a — 635.6 — 635.6747.7 of Example 1b 598.2 — 598.2 — — Semicrystalline polyester ofExample 2a — 112.2 — — — of Example 2b 149.5 — — — — of Example 2e⁽¹⁾ —— 149.5 112.2 — Crosslinking agent A⁽²⁾ 56.3 56.3 56.3 56.3 56.3 Blackiron oxide⁽³⁾ 45 45 45 45 45 Black iron oxide⁽⁴⁾ 140 140 140 140 140Carbon black⁽⁵⁾ 11 11 11 11 11 Flow-regulating agent⁽⁶⁾ 10 10 10 10 10Benzoin 3.5 3.5 3.5 3.5 3.5 Properties 60° gloss 32 44 85 90 87 Impactresistance (kg · cm) direct 100 180 120 60 0 reverse 100 200 120 40 0Pencil hardness H 2H H 2H 2H ⁽¹⁾by way of comparison ⁽²⁾triglycidylisocyanurate (ARALDITE PT 810 by Ciba-Geigy) ⁽³⁾BAYFERROX 130 (Bayer)⁽⁴⁾BAYFERROX 3950 (Bayer) ⁽⁵⁾FW 2 (Degussa) ⁽⁶⁾RESIFLOW PV5 (WorléeChemie)

Table II shows that the compositions according to the invention(Examples 16 and 17) give low-gloss coatings while the compositions notaccording to the invention (Comparative Examples IV and V), whichcontain a semicrystalline polyester essentially based on terephthalicacid and 1,6-hexanediol, give very glossy coatings (gloss of 85 to 90%).

EXAMPLES 18 Weatherability of the Coatings.

In this example, the excellent weather behaviour of the low-glosscoatings obtained from compositions according to the invention isdemonstrated.

For this purpose, the coatings obtained with the dark brown paintsprepared in Examples 16 and 17 and Comparative Examples IV, V and VI(the compositions of which are given in Table II) were tested withrespect to their weatherability.

The powder paints to be tested are deposited by spraying them using aGema-Volstatic PCG 1 electro-static spray gun onto chromium-platedaluminium panels under a voltage of 60 to 100 kV. The thickness of thecoating deposited is from 50 to 70 micrometers.

After curing for 15 minutes at 200° C., the coatings obtained aresubjected to the accelerated ageing test (the QUV test described above)so as to determine the UV resistance and moisture resistance. The colourchange delta E was determined according to the ASTM D 2244 standard andthe retention of the gloss, measured at an angle of 60° C., wasdetermined according to the ASTM D 523 standard after 3000 hours'exposure.

The results thus obtained are given in Table III in which:

first column indicates the composition tested

second column the colour change delta E calculated according to the ASTMD 2244 standard after 3000 hours' exposure third column the retention ofthe gloss, measured at an angle of 60° C., according to the ASTM D 523standard after 3000 hours' exposure and expressed as a percentage of itsinitial value at the start of the experiment.

TABLE III Accelerated ageing of the coatings Composition Delta ERetention of the gloss (%) of Example 16 5.3 63 of Example 17 3.5 82 ofExample IV⁽¹⁾ 16.8 32 of Example V⁽¹⁾ 15.1 41 of Example VI⁽¹⁾ 4.8 92⁽¹⁾by way of comparison.

The results in Table III show that the low-gloss coatings obtained fromthe compositions according to the invention exhibit remarkably goodweatherability (Examples 16 and 17).

This weatherability is comparable to that obtained with compositionsbased on amorphous polyesters rich in isophthalic acid that arecommercially available and reknowned for their excellent outdoorexposure performance (Comparative Example VI). The presence of thesemicrystalline polyester based on 1,12-dodecanedioic acid and1,6-hexanediol in the compositions according to the invention hardlyaffects the weatherability of the cured coatings obtained.

It may be seen in Table III that the same does not apply with regard tothe compositions of the prior art which contain a semicrystallinepolyester of the type of those used in the illustrative embodiments ofEuropean Patent 521,992, which are based essentially on terephthalicacid and 1,6-hexanediol. This is because the coatings obtained withthese compositions are extensively damaged by exposing them to UV lampsand to moisture: the colour change delta E is very significant and thegloss falls below 50% of its initial value after 3000 hours' exposure(Comparative Examples IV and V).

What is claimed is:
 1. A thermosetting powder coating composition that,when cured, results in a coating having a gloss of less than 50%measured at an angle of 60° according to ASTM D 523 standard, whereinthe composition comprises a binder, wherein the binder comprises a blendof (a) an amorphous polyester containing carboxyl groups, rich inisophthalic acid, prepared from an acid constituent comprising from 55to 100 mol % of isophthalic acid, from 0 to 45 mol % of at least onedicarboxylic acid other than isophthalic acid and from 0 to 10 mol % ofa polycarboxylic acid containing at least 3 carboxyl groups and from analcohol constituent comprising from 60 to 100 mol % of neopentylglycol,from 0 to 40 mol % of at least one dihydroxylated compound other thanneopentylglycol and from 0 to 10 mol % of a polyhydroxylated compoundcontaining at least 3 hydroxyl groups, the said amorphous polyesterhaving a glass transition temperature (T_(g)) of at least 50° C. and anacid number of 15 to 100 mg of (b) a semicrystalline polyestercontaining carboxyl groups prepared either (b1) from 1,12-dodecanedioicacid and from a saturated aliphatic diol with a linear chain having from2 to 16 carbon atoms and optionally from a polycarboxylic acidcontaining at least 3 carboxyl groups or from a polyol containing atleast 3 hydroxyl groups, or (b2) from 40 to 100 mol % of1,12-dodecanedioic acid and from 0 to 60 mol % of an aliphaticdicarboxylic acid with a linear chain having from 4 to 9 carbon atoms,calculated with respect to the total of the dicarboxylic acids, from acycloaliphatic diol having from 3 to 16 carbon atoms and optionally froma polycarboxylic acid having at least 3 carboxyl groups or from a polyolhaving at least 3 hydroxyl groups, the said semicrystalline polyesterhaving a melting point (T_(m)) of at least 40° C. and an acid number of5 to 50 mg of KOH/g; and (c) a crosslinking agent.
 2. A compositionaccording to claim 1, wherein, the amorphous polyester (a) has a glasstransition temperature of 50 to 80° C. and an acid number of 30 to 70 mgof KOH per gram.
 3. A composition according to claim 1, wherein theamorphous polyester (a) has at least one of the followingcharacteristics: a number-average molecular weight of between 1100 and11,500 and a melt viscosity of 100 to 15,000 mPa·s at 200° C.
 4. Acomposition according to claim 1, wherein the acid constituent of theamorphous polyester (a) contains, apart from isophthalic acid, up to 45mol % of a dicarboxylic acid chosen from terephthalic acid, phthalicacid, succinic acid, glutaric acid, adipic acid, pimelic acid, subericacid, azelaic acid, sebacic acid, fumaric acid, maleic acid,1,3-cyclohexanedicarboxylic acid, 1,4-cyclohexanedicarboxylic acid andmixtures of these compounds.
 5. A composition according to claim 1,wherein the alcohol constituent of the amorphous polyester (a) contains,apart from neopentylglycol, up to 40 mol % of a dihydroxylated compoundchosen from ethylene glycol, propylene glycol, 1,4-butanediol,1,6-hexanediol, 2-methyl-1, 3-propanediol,2-butyl-2-ethyl-1,3-propanediol, 1,4-cyclohexanedimethanol, hydrogenatedbisphenol A, neopentylglycol hydroxypivalate and mixtures of thesecompounds.
 6. A composition according to claim 1, wherein thesemicrystalline polyester (b) has a melting point of between 40 and 90°C., an acid number of 5 to 30 mg of KOH per gram and a hydroxyl numberwhich does not exceed 5 mg of KOH per gram.
 7. A composition accordingto claim 1, wherein the semicrystalline polyester (b) has at least oneof the following characteristics: a number-average molecular weight ofbetween 2200 and 25,000 and a melt viscosity of from 50 mPa·s at 100° C.to 10,000 mPa·s at 150° C.
 8. A composition according to claim 1,wherein the semicrystalline polyester (b1) is the polyesterificationproduct of 1,12-dodecanedioic acid with 1,6-hexanediol.
 9. A compositionaccording to claim 1, wherein the semicrystalline polyester (b2) is thepolyesterification product of 1,12-dodecanedioic acid with1,4-cyclohexanediol or 1,4-cyclohexanedimethanol.
 10. A compositionaccording to claim 1, wherein the amount of amorphous polyester (a)represents approximately 60 to 87% by weight and the amount ofsemicrystalline polyester (b) represents approximately 13 to 40% byweight calculated with respect to the total weight of the amorphouspolyester (a) and of the semicrystalline polyester (b).
 11. Acomposition according to claim 1, wherein the amount of amorphouspolyester represents 60 to 82% by weight and the amount ofsemicrystalline polyester (b) represents 18 to 40% by weight calculatedwith respect to the total weight of the amorphous polyester (a) and ofthe semicrystalline polyester (b).
 12. A composition according to claim1, wherein the crosslinking agent (c) is a polyepoxidized compound. 13.A composition according to claim 1, wherein the crosslinking agent (c)is a beta-hydroxyalkylamide.
 14. A composition according to claim 1,wherein the ratio of the amount of amorphous polyester (a) and ofsemicrystalline polyester (b) containing carboxyl groups, on the onehand, to the amount of crosslinking agent (c), on the other hand, issuch that there are from 0.5 to 1.5 equivalents of carboxyl groups perequivalent of functional groups in the crosslinking agent.
 15. Acomposition according to claim 1, wherein the crosslinking agent (c) ispresent in an amount of approximately 4 to 25% by weight calculated withrespect to the total weight of the binder.
 16. A composition accordingto claim 1, wherein the binder comprises from 45 to 83% by weight ofamorphous polyester (a), from 13 to 30% by weight of semicrystallinepolyester (b) and from 4 to 25% by weight of crosslinking agent (c). 17.A composition according to claim 1, wherein the binder comprises from 45to 78% by weight of amorphous polyester (a), from 18 to 30% by weight ofsemicrystalline polyester (b) and 4 to 25% by weight of crosslinkingagent (c).
 18. A varnish or paint, comprising a thermosetting powdercomposition according to claim 1, together with auxiliary substancesacceptable for use in the powder varnish or paint.
 19. A satin or mattcoating, comprising a cured thermosetting powder composition accordingto claim
 1. 20. A process for obtaining a low-gloss coating on anarticle, which comprises applying a thermosetting powder coatingcomposition according to claim 1 to the article and curing the articlethus coated at a temperature of 140 to 200° C.
 21. An article entirelyor partially coated using the process according to claim
 20. 22. Aprocess according to claim 20, wherein the article thus coated is curedat a temperature of 140 to 200° C. for up to 30 minutes.